There has been conventionally known a fuel vapor processing apparatus that causes a canister temporarily to adsorb fuel vapor generated in a fuel tank and introduces the fuel vapor desorbed from the canister as required into an intake passage of an internal combustion engine to purge the fuel vapor. As one kind of fuel vapor processing apparatus like this, a fuel vapor processing apparatus that measures the concentration of fuel vapor in an air-fuel mixture introduced into an intake passage before the fuel vapor is purged and can hence purge a large quantity of fuel vapor in a short time is disclosed in patent documents 1, 2. In the fuel vapor processing apparatus disclosed in such patent documents 1, 2, the flow rate or the density of the air-fuel mixture in a passage for introducing fuel vapor into an intake passage is detected and the flow rate or the density of air in a passage open to the atmosphere is detected and the concentration of fuel vapor is computed from the ratio of these measurement results.
[Patent document 1] JP-5-18326A
[Patent document 2] JP-6-101534A
In fuel vapor processing apparatuses disclosed in the patent documents 1, 2, negative pressure in the intake passage is applied to respective passages to pass the air-fuel mixture or air through the respective passages and at the same time the flow rate or the density of the air-fuel mixture or air is detected. Therefore, when the negative pressure pulses, the flow rate or the density fluctuates and hence the concentration of fuel vapor computed on the basis of the detection results of such flow rate or density deteriorates in accuracy. Moreover, when the negative pressure in the intake passage is small, the flow rate of the air-fuel mixture or air in the respective passages decreases and hence cannot detect the flow rate or the density of the air-fuel mixture or air.
Therefore, the present inventors have earnestly conducted research on a fuel vapor processing apparatus that reduces pressure in a detection passage and passes air and an air-fuel mixture through the detection passage and at the same time monitors a change in pressure and computes the concentration of fuel vapor on the basis of the monitoring results. In such a fuel vapor processing apparatus, because pressure in the detection passage is reduced by a pump, a pressure to be detected is made stable except when detection conditions are changed and the flow rate of air or air-fuel mixture can be sufficiently secured in the detection passage. However, the results of research further conducted by the present inventors revealed that in a case where a pressure was detected while an air-fuel mixture was passing through the detection passage, when the air-fuel mixture taken into the detection passage by the pressure reducing action of a pump was sucked into the pump, the detection result of pressure fluctuated for some time. This problem is thought to be caused by the fact that the characteristics of the pump vary before and after the air-fuel mixture reaches the pump and hence can be solved when the pressure is detected after the characteristics of the pump stabilize. However, when the pressure is detected after the characteristics of the pump stabilize, the total time required to measure the concentration of fuel vapor increases and hence time for purging after the measurement of the concentration decreases. This presents a new problem that the quantity of actual purge (hereinafter referred to as “actual quantity of purge”) decreases.